1. Field of the Invention
The present invention relates generally to a speech recognition system for an automotive vehicle, and more particularly to a speech recognition system by which a driver's spoken instructions can be reliably recorded or recognized even when engine noise increases within the passenger compartment after the vehicle engine begins to operate.
2. Description of the Prior Art
There is a well-known speech recognizer which can activate various actuators in response to human spoken instructions. When this speech recognizer is mounted on a vehicle, the headlight, for instance, can be turned on or off in response to spoken instructions such as "Headlight on" or "Headlight off". Such a speech recognizer usually can recognize various spoken instructions in order to control various actuators; however, there are some problems involved in applying this system to an automotive vehicle.
Usually, the speech recognizer is used in a relatively quiet environment; however, the speech recognition system for an automotive vehicle is usually used within a relatively noisy passenger compartment and additionally the noise fluctuates intensely therewithin. Therefore, one of the major problems is how to cope with erroneous spoken phrase recordings or recognitions caused by fluctuating engine noise within the passenger compartment.
In the prior-art speech recognizer, since a spoken instruction signal including noise is always amplified on a constant gain factor, when the noise level within the passenger compartment increases, especially when the engine begins to operate and therefore the engine noise is inputted to the speech recognizer at random, the noise mixed with the spoken instruction signal at a relatively high ratio is also amplified together with the spoken instruction signal, thus causing a problem in that the spoken instruction cannot be recognized reliably or is recognized erroneously to operate a wrong vehicle device actuator.
Furthermore, in order to distinguish a spoken instruction from noise, conventionally there is provided a voice detector in the speech recognizer, by which the start and the end of a spoken instruction are determined by detecting whether the magnitude of a spoken instruction signal exceeds a predetermined reference threshold voltage level for a predetermined period of time or whether the magnitude of the spoken instruction signal drops below the predetermined reference threshold voltage level for another predetermined period of time, respectively.
In the prior-art speech recognizer, however, since the reference threshold voltage level is fixed, when noise signal level is high, for instance, when the vehicle is running and therefore the noise level exceeds the reference threshold voltage level for a long time, there exists a problem in that the voice detector can erroneously consider this state to represent the beginning of a spoken instruction. In other words, the prior-art speech recognizer is prone to erroneous recognition due to intense noise within the passenger compartment.
A more detailed description of a typical prior-art speech recognizer and a typical prior-art voice detector will be made with reference to the attached drawings in conjunction with the present invention under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.